Large aperture telephoto objective



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LARGE APERTURE TELEPHOTO OBJECTIVE 3 Sheets-Sheet l T Filed Nov. 6, 1947BY M45 A Troie/v6# Aug 3, 1954 J. G. BAKER LARGE APERTURE TELEPHoToOBJECTIVE 3 Sheets-Sheet 2 Filed NOV. 6. 1947 INVENTOR. .A4/5 34a/fe lBYm ATTOQ/VEY t ,Mawr- Aug. 3, 1954 J. G. BAKER LARGE APERTURE TELEPHoToOBJECTIVE 3 Sheets-Sheet 3 Filed NOV. 6, 1947 Mae.,

Patented Aug. 3, 1954 LARGE APERTURE TELEPHOTO OBJECTIVE James G. Baker,Waban, Mass., assignor to the United States of America as represented bythe Secretary of War Application November 6, 1947, Serial No. 784,478

8 Claims. l

This invention relates to large aperture photographic objectives andparticularly to telephoto objective lens systems.

It is a primary object of this invention to provide a telephotoobjective having a large aperture and long equivalent focal length.

It is another important object of this invention to provide a lenssystem for telephoto objectives having an air spaced triplet using ahigh index flint negative element between two low index positive crownelements in the objective component and having air spaced flint positiveelements preceding negative crown elements in the rear component toeliminate distortion without impairing definition at maximum telephotoeffect on a flat eld.

It is a further object of this invention to provide a compactglenssystem for telephoto objectives of large aperture and long equivalentfocal length by reecting the light beam on mirrored surfaces.

There has been an expressed need for long equivalent focal lengthtelephoto objectives with a low telephoto ratio. The telephoto ratio asused in this disclosure refers to the ratio of the camera length asmeasured between the focal plane and the vertex of the front lenssurface, and the equivalent focal length. Such telephoto objectives haveinnumerable uses and are particularly desirable for aerial photographyWhere a high degree of resolution and contrast are necessary forprecision reconnaissance, and the like. In usual practice, lens formsthat have been found to perform satisfactorily in small focal lengthshave been scaled up and modied for use in aerial photography at largefocal lengths. However, this practice has been found to lead tophotographs of lowered linear resolution and lowered contrast that inpart defeat the purpose of using large focal lengths. Lenses of largefocal length must therefore employ improvements in design beyond therequirements of scaling if satisfactory results are to be achieved. Atthe same time in order to avoid cumbersome and weighty construction oflarge lens systems it has proved advisable to make use of the telephototype of optical system, a choice that makes the burden on the opticaldesign doubly difficult. In the present invention a type of telephotosystem has been devised that obviates all the above-mentioneddifficulties and provides satisfactory results at large focal lengths.

The present invention provides a lens system for large aperturetelephoto objectives which produce good resolution and definition atmaximum telephoto effect on a fiat field that is distortionless and issubstantially free of undesirable aberrations. This is accomplished byusing an airspaced triplet consisting of a high index flint negativeelement between two low index crown positive elements for the forwardobjective component and air spacing one or both of two doublets in therear component in which a positive flint element may precede a negativecrown element in each doublet. The use of the low index crown and a highindex int in the forward objective component, contrary to the usualpractice in standard lenses, favors the telephoto effeet, permitsdistortion correction and provides the elimination of zonal terms in thesine condition. However, the above elements tend to increase lateralcolor which is restricted by the lter and by the restricted angularcoverage of these large lenses.

In the construction of the lens system, placing a filter in theconverging beam behind the last element is contemplated. The aberrationsof the filter are small and can be taken into account in the lensdesign. Positioning the filter behind the lens system has severaladvantages in that the surface gure need be only a fraction as good as alter in front of the lens system; the lter diameter need be only abouthalf as large which simplifies manufacture; and the filter thickness canbe used to adjust the best focus in any color that goes with the filter,that is, an infra-red filter should be thinner than the yellow filter inorder that the infra-red focus lie on the focal plane determined foryellow light.

Referring to the drawings:

Fig. 1 shows a telephoto objective lens system in accordance with theinvention Fig. 2 shows another embodiment of the invention; and

Fig. 3 shows a modification of the lens system shown in Fig. l with theray path reflected on mirrored surfaces.

Referring to Fig. 1, which illustrates the invention, there is shown byway ofl example a telephoto objective lens system that consists oflenses I to VII, inclusive, having radii of curvature represented by R1to R16, inclusive; lens thicknesses represented by D1 to Da, inclusive;and air spaces represented by S1 to Ss, inclusive. The lenses I, II, andIII are on the side of the longer conjugate and will be referred to asthe forward objective component; while the lenses IV to VII, inclusive,are on the side of the shorter conjugate and will be referred to as therear component.

The element VIII is a filter of 'plano-plano construction that is placedin the converging beam of the last element with respect to the directionof light travel through the system. The index of refraction andthickness of this filter must be considered in the lens system and is 3given in the examples of Fig. 1 although the iilter may be changed fordiiierent light.

Two examples of specifications are given below in accordance with theinvention. Example 1 provides a lens system of iO-inch equivalent focallength and a relative aperture of f/5. The notation R1, Ra etc. refersto the successive radii of curvature, counting from the front, whichwill be denoted as convex toward the longer conjugate in the absence ofany sign, and will be denoted by a minus sign when the radii ofcurvature is concave toward the same, as is conventional. Nn is used inthe examples to denote the refractive indices of the lens elements forthe D-line and the Abbe-V number is used to denote the dispersion value.The radii, thicknesses and spacing of the lens elements for these twoexamples, and all other examples in this disclosure, are given ininches. The refractive indices and Abbe-V numbers of Example 2 are thesame as for Example 1. As may be noted in Example 2 the lens thicknessesand radii of 5 curvature as well as the air spaces of Example 1 10 ducedin practicing the invention of Example 1.

By scaling up or down the lens thicknesses, radii of curvature, and airspaces, and by altering these quantities by small amounts according tothe requirements of optical design, one can produce 15 telephotoobjective lens systems of various focal lengths, relative apertures, andangular coverages without departing from the spirit and scope of thisinvention.

Example 1 [F=40 inches i/] Thickness S ace Abbo-V El Rad (inches)(inches) (inrhes) Nd Number R1 Il 14. 058 I D11'. 882 1. 5168 64. 5

Sl- .264 Rx 33.942 II Dif-.405 1.7195 29.3

Sg- .051 Rl l 7. 664 m Dx". 941 1. 517 64. 5

Re l 19. 722

S;- 9.454 R1 Il 40.066 IV D4-. 395 1. 6164 36. 6

Sul .819 Rg Il 5.724 v DIU. 356 1. 6102 58.8

Rxo- 14.14

Sl- 3.002 R11- 11.254 VI Dlll. 499 1. 6164 36.

Rn'- 17.347 VII DFI-407 1. 6102 58.8

B1- .916 R15-Plano VIII. Dn. 523 1. 517 Y7, R-2

Ril-P18110 Example 8 [F- inch f/5] E1. man (mene.) 'lfxlgg (gigs) N.Abbo-v Number R1 Il 2,1. 525 I D11-1.351 1. 5168 64.5.

Sx- .401 Ra l 51.436 II Dz- .614 1. 7195 29.3.

Sa- .077 R5 11.649 m Dl*1 144 1. 517 64.5.

5;-14. 677 R1 l 60. 73 Iv D4- .599 1.6164 36.6.

S4- 1.241 R '-8. 670 v D6* .542 1.6102 58.8.

B5- 4. 550 R11- 17.704 VI. D;- 732 1. 6164 36.6.

Sg# 1.288 R15-l 32. 374 VII. D11-'1. 573 1. 6102 58.8.

' B1- 1. 418 Ril-Plano vm. Dl- 541 l. 517 Grange Film.

It may be noted that the element D: in Example 2 has a. large centralthickness that is not ordinarily necessary for the optical requirementsot the 60inch, f/5 telephoto objective. This great thickness isintroduced into the design of this telephoto objective for the purposeof particularly adapting this optical system for high altitude ilying inwhich the optical system is evacuated. The element Dn forms one boundaryof the evacuated lens system and is subjected to great pressure whichpressure depends on altitude. The other boundary of the vacuum spacecontaining the optical system is formed, in actual practice, by aplane-parallel window of considerable thickness but of no opticaleffects and therefore has not been shown in the optical system.

Referring to Fig. 2 there is shown a modification o! the lens system ofFig. 1 which is a abl infra-red tele hoto use. Reference is rected to eescriptive c ar below in which Referring to Fig. 3 there is shown amodication of the lens system of Fig. 1 in which two front surfacemirrors M1 and Mz are used to direct the light rays in a U-shaped path.In this manner the camera in which this lens system may be used can bemade more compact. A full description of this lens system is givenbelow. The descriptive example given in this chart will provide a60-inch f/6 telephoto objective but the specifications of lensthicknesses, radii of curva.- ture, and air spaces may be scaled up ordown for other focal lengths and relative apertures, if desired, withoutdeparting from the spirit and scope of this invention.

[F=60lnches f/] Th1 kms Distance Abb6v El. Radiianches) (memes) (Inches)N5 Number S1- .261 C.- -47.96s B.. t.- .744 1.72 29.3.

Sl- .061 0|- 11.456 o ti- .954 1.517 64.5.

S|==14.451 C1 59.524 D n- .695 1.617 36.6

s.- 1.122 -ov- 9.196 E -zi- .595 1 6106s 57.2

S5- 6.977 0n- 18.572 F-.. t.- .695 1.617 36.6.

S5- .726 c11- -25819 i G 11- .601 151065 57.2.

s1- .595 C15-'Plano H t.- .496 1.511 R, or orange ciu-Plano Filter.

[F-io mem 5] The rear element of the rear component of the 60modications shown in Figs. 2 and 3 could be Thickn s s am Abbeyincreased in thickness to adapt these optical sys- El. RadiiClncheS)(Inche (111,511.18) Nd bigltems for evacuated high altitude systems byaltering the rear component in accordance with r 13 387 optical design,if desirable. 1 d,=,g66 1 5168 6.1 5 65 Many advantages have beenattained by the r,=141.6a s a 261 above described telephoto objectivesover other f, 34 003 known systems. The low optical power and form 2 r34 274 d1=-396 1-7195 29-3 of the rear component make these telephotos,= .05o objective systems suitable for incorporating tem- 3 fl 7-54 d z924 1 5164 64 5 70. perature and air density compensation and the 19,699i' rear elements of each is particularly suitable for r 3884 SF 9'317acting as one boundary of an evacuated lens 4 1 d=.3s7 1.6192 58.8system, illustrated in Fig. 1, Example 2. The fl* -58-074 Sw 810 `choiceof glass types and lens forms brought about fl l 5.580 75 the fact thatthe negative rear component o! four lenses lies at an unusually lowheight in the converging beam making possible the use of a. relativelysmall iris diaphragm and a betweenthe-lens shutter of useful size forcamera opera.- tion. These telephoto objective systems each provide verygood resolving power and permit faster shutter speeds which make themextremely useful for aerial photography.

The 60-inch f/6 telephoto objective lens system, using mirrors to directthe rays in a reverse direc- [F-io inches. i/s] Thickness Space Abbe-VEl' Rad (Inches) (inches) (Inches) Nd Number R1 14.058 I Di=. 882 l.5168 64. 5

Rz l'-115. 80

S1= 264 Ri 33. 942 II D1=. 405 1. 7195 29. 3

Si: 051 Rs 7. 664 III Da=. 941 1. 517 64. 5

S3=9. 454 R1 40. 066 IV D4=. 395 1. 6164 36. 6

S= 819 Re 5. 724 V D5=. 356 1. 6102 58. B

Rio= 14. 164

S5=3. 002 Ru= 11. 254 VI D=l. 499 1. 6164 36. 6

Riz= 37. 075

Sa= 850 Ria= 17. 347 VII :Dr-I. 407 1. 6102 58. 8

S1 916 R15=Plano VIII Dg-I. 523 1. 517 Y7, R-2

R|=P1ano f Sa=1l. 631 l tion, has the advantage of providing a compactcamera, particularly for aircraft where space is usually at a premium,without sacrificing any of the advantages of the systems of Fig. 1 and2. AWhile preferred forms of the invention have been shown anddescribed, it is to be understood 2. A telephoto objective havingnumerical data 0 as set forth in the following chart where R1, Rz,

that modmcations and changes may be made 4.-, air separations, Ndrepresents the indices of rewithout departing from the spirit and scopeof this invention and I desire to be limited only by the scope of theappended claims.

fraction, the Abbe-V number represents the dispersion values, and Frepresents the equivalent focal length:

Thickness Space Abbe-V El. Radi! (Inches) (Inches) (Inches) N .i NumberR1 l 21.525 L Di=1.351 1.5168 64.5.

Si .401 Rl l 5L436 II Dz- .614 1.7195 29.3.

Si .077 R; =l 11.649 III DPI. 144 1. 517 64.5.

Rc Il 30.121

Sa=14.677 R1 I 60.72 IV D4== .599 1.6164 36.6.

S4- 1.241 R- 8. 670 V D|== .542 1.6102 58.8.

Rin* 21.442

Sl. 4.550 Ru- 17.704 VI Dl* .732 1. 6164 36.6.

So- 1.288 Rn- 32.374 VII D1=1.573 1.6102 58.8.

S11- 1. 418 Ris-Plano VIII Di- .541 1.517 Orange Filter.

RuPIBllO represent the axial 5. A large aperture telephoto objectivelens system comprising; a convergent front objective component having afocal point on the side of the shorter conjugate and having the frontand of the elements, s1. s2, represent the axial 5 rear surfaces thereofconvex toward the longer air separations, Nd represents the indices ofreconjugate to produce a, second principal plane fraction, the Abbe-Vnumber represents the disforwardly of the center ofA said objectivecompersion values, and F represents the equivalent ponent; and a reardivergent eOmDOrlen Speeed focal length: on the side of the shorterconjugate of said front [hm inches 5] 10 objective component not lessthan 1A; or more than EA the front objective component focal Abb Vlength and having a focal point on the side of the Thickness space flonger conjugate in front of said front objective E1. R d1 In 11 N N a Ic es) (Inches) (Inches) d le? component a distance not more than 4 orless than 2 times the axial distance along the optin 13.367 cal axisbetween the two components, said rear 1 r, 141'63 d* :'866 1'5168 64'5divergent component including at least two 34 003 s1= .261 doublets onthe side of the front convergent 2 f (1,: 393 1,7195 29.3 component eachhaving a positive element pren 34.274 s o ceding a negative element inthe order named ,5: 7.54 from said front objective component and being r19 699 d1 =.924 1.6164 64.5 spaced apart a distance not less than 1/4 ormore s.: 9 317 than 1/2 the axial distance along the optical axis 4 f13&84 d 387 l 6102 58 8 between the two said components, one element n:58614 25 of each doublet being flint and the other ele- 5 580 S= -810ment of each doublet being crown, the crown ele- 5 d5 149 1,6164 36,6ments having the same index of refraction and f1= 134309 Si: 3 162 thesame dispersive value and the flint elements r= 16.91 having the sameindex of refraction and the same 6 m: 23 227 d =513 1'6164 3616 30dispersive value, the index of refraction of the 7 d1 =.15o 1.6102 58.8last-mentioned flint elements being greater than "l: 35-941 S6: 513 1.61with a dispersion value of less than 40 and r11= 17. 772 the index ofrefraction of the crown elements be- 8 m: Plano d =-150 1-6102 58-8 ingless than 1.61 with a dispersion value greater 9 d, 513 1.6164 36.5 35than 55, the front surface of each doublet and the m: 4s-873 10:.1501.61908 6M rear surface of the rearmost doublet being con- 1o m= 37.189vex toward the longer conjugate to establish in 51:12' 746 said reardivergent component together with said l front objective component asecond principal 4. A telephoto obiectivehavmg numerical data 4o pianefor the lens system forward of said front aS Set forth in the fOllOWlngChart Where C1, C2, objective component and a focal plane for therepresent the radii Of eurvetllre 0f the Sllrlens system rearwardly ofthe rear divergent feces. t1. t2, represent the axial thicknessescomponent such that the axial distance along of the elements, 11.12,represent the axial air the optical axis from the front vertex of the0bseparations, N5 represents the indices of refrac- 45 jective componentto said focal plane of the lens tion, the Abbe-V number represents thedispersystem is less than 82% of the distance from said sion values, andF represents the equivalent focal second principal plane of the lenssystem to said length: focal plane of said lens system, and the distance[F= inches f/] El Radll Thickness Distance Nd Abbe-V (Inches) (Inches)(Inches) Number cl 24. 521 A t1=1. 055 1. 517 64.5.

s1= .261 C3 47.038 B 1= .744 1.72 29.3.

s== .061 c1= 11.456 c 11= .954 1.517 64.5.

s1=14 451 o1= 59.524 D t= .695 1. 617 36.6.

s.- 1.122 c5= 9.196 E t1= .695 1.61669 57.2.

s.- .726 C11= 25. 819 G t1- .601 1 6106s 57.2.

clp 63.227 C Plano S1=l .595 H l .496 1.517 R1 61- orange C15=P11m6Futer.

from the front vertex of said front objective component to the focalplane of the lens system is not more than 3 or less than 2 times thedistance from the rearmost vertex of the rear divergent component to thefocal plane of the lens system to provide a large aperture telephotoobjective of relative long equivalent focal length that isdistortionless and substantially free from undesirable aberrations toproduce good resolution and denition on a flat field.

6. A large aperture telephoto objective lens system as set forth inclaim 5 wherein the forward doublet of said rear divergent component hasthe positive and negative elements thereof air spaced an axial distancealong the optical axis not less than 1/15 or greater than 116 the axialdistance along the optical axis between said two components and forminga centrally thick air lens therebetween.

'7. =A large aperture telephoto objective lens u system comprising; aconvergent front objective f members on the side of the tripletcomponent the triple component consisting of a negative high index dintelement coaxially air spaced between two positive low index crownelements, the foremost air space characterizing a centrally thickmeniscus air lens concave toward the longer conjugate and the rearmostair space characterizing a centrally thin meniscus air lens convextoward the longer conjugate, said two low index crown elements havingthe same index of refraction and the same dispersion value, the index ofrefraction thereof being lower than 1.52 with the dispersion valuethereof being greater than 60 and the index of refraction of said highindex flint element being greater than 1.71 with a dispersion value lessthan 30, the powers of said positive crown elements being greater thanthe power of said negative flint element to produce its focus on theoptical axis thereof on the side of the shorter conjugate, and the frontand rear surfaces of said triplet component being convex toward thelonger conjugate to establish a second principal plane forwardly of thecenter of said triplet component; and a divergent rear cornponent spacedalong'the optical axis on the side of the shorter conjugate of saidtriplet component not more than 1%, or less than V3 of the focal lengthofwsavd triplet component,|said divergent `."'rea'r componitcl'tdmg easttwo doublet doublet members of the divergent rear components each havinga positive element preceding a negative element in the order named fromthe longer conjugate with one element of each doublet being ilint andone element of each doublet I being crown, the crown elements having thesame index of refraction and the same dispersion value and the iiintelements having the same index of i refraction and the same dispersionvalue, the index of refraction of the last mentioned iiint elementsbeing greater than 1.61 with a dispersion value l2 ,less than and theindex of refraction of the fcrcwn elements being less than 1.61 with adisfpersive value greater than 55, said doublets being separated by morethan f4 and less than 1/2 the axial distance along the optical axisbetween the said triplet component and said rear divergent component,the front surface of the forward doublet and the front and rear surfaceof the rearward doublet being convex toward the longer con- "jugate toproduce in combination in said rear l,divergent component a secondprincipal plane #therefor forward of the center of said rear divergentcomponent and to produce a focal point on the side of the longerconjugate forward of said triplet component not more than 4 or less than2 times the distance of said triplet component from said rear divergentcomponent to establish in the combination of said rear divergent:component and said triplet component a second principal plane forwardof said triplet component and a focal plane rearwardly of said reardivergent component, the axial distance along the optical axis from thefront vertex of said triplet component to said focal plane of said lenssystem being not less than 2 or more than 3 times the axial distancealong the optical axis from the rear vertex of said rear divergentcomponent to said focal plane of the lens system which constitutes thetelephoto effect of the lens system, and the axial distance along theoptical axis from the front vertex of said objective triplet componentto said focal plane of said lens system being not more than 82% of thedistance from said second principal plane of the lens system to saidfocal plane of the lens system constituting the telephoto ratio of thelens system whereby to provide a large aperture telephoto objective oflong equivikalent focal length.

8. A large aperture telephoto objective lens system as set forth inclaim 7 wherein the foremost doublet of said rear divergent componenthas the positive and negative elements thereof air spaced an axialdistance along the optical axis not less than 1,(,5 or greater than 116the axial distance along the optical axis between said two components,said air space forming a centrally thick air lens.

References Cited in the le of this patent UNITED STATES PATENTS NumberName Date 1,791,276 Konig Feb. 3, 1931 2,378,170 Aklin June 12, 1945FOREIGN PATENTS Number Country Date 4,523 Great Britain of 1906 7,229Great Britain of 1913 222,709 Great Britain Oct. 9, 1924 471,565 GermanyFeb. 15, 1929 388,215 Great Britain Feb. 2'3, 1933

